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Sunday, October 20, 2013

I’ve seen the evolution of computers used as an analogy for the
evolution of spacecraft. Once, all
computers were massive machines (called mainframes). Then the evolution of electronics allowed the
creation of what were known as mini-computers that expanded the number of
organizations that could afford the computer.
Further evolution brought the personal computer to billions of people,
and now the smart phone allows hundreds of millions of use to carry computers
in our pockets.

Similarly, planetary spacecraft originally tended to be large missions
(today called Flagships). NASA’s New
Frontiers and Discovery program enabled a series of lower cost missions that
can be compared to mini-computers. As I
discussed in an August
post, planetary SmallSats could become the equivalent of personal
computers.

Before the decade is out, I expect that we will have at least one
planetary mission based on the equivalent of the smartphone, a CubeSat.

Comparison of masses and costs of missions with different classes of spacecraft. Click on the image for a larger view.

CubeSats
were invented a decade ago by researchers at the California Polytechnic State
University (CalPoly) and Stanford University to create a standard for
university-built spacecraft. As the name
suggests, the basic configuration is a cube.
At just 10 centimeters on a side (or 1unit (1U) in CubeSat parlance) and
weighing approximately one kilogram, CubeSats depend on the electronics
miniaturization that has enabled personal computers and smartphones. If a research team needs a larger spacecraft,
cubes can be stacked together to create larger volumes with standard
configurations up to six units and even twelve units. The small size of CubeSats allows them to be
carried into orbit cheaply as secondary payloads on launches of larger Earth
satellites.

Given the success of Earth-orbiting CubeSats, it’s only natural that
engineers and researchers want to use their design philosophy to explore beyond
Earth. There’s even a regular conference devoted to the topic.

I don’t doubt that CubeSats will study other worlds in the coming
decade. However, there are a number of
challenges designers will have to overcome: Longer lifetimes (years instead of
weeks or months), propulsion for trajectory changes and potentially entering
orbit, power (especially further from the sun), communications from vastly
further distances than Earth orbit, higher radiation outside Earth’s protective
magnetosphere, and instruments to make meaningful measurements. The technology to enable serious planetary
CubeSat spacecraft is under development but not yet here. . (This
presentation gives a good overview of technology in development while this
presentation shows the vision for one approach to interplanetary CubeSats.)

The first CubeSat mission planned to leave Earth orbit is a NASA-funded
project led by the Jet Propulsion Laboratory with several university partners. Called INSPIRE
(for Interplanetary NanoSpacecraft Pathfinder In a Relevant Environment),
the twin spacecraft will each be 3.8 kg and 3U in size. The mission goals are modest: travel
150,000,000 km from Earth (in an orbit near the Earth’s around the sun),
enhance CubeSat design to successfully operate outside Earth orbit, and conduct
scientific measurements with a magnetometer and simple camera, and communicate
with the Deep Space network. The
spacecraft should be ready launch in 2014.

Each INSPIRE spacecraft will generate just 20 watts of power
using body mounted solar arrays. A
downside to this approach is that at least half the spacecraft’s body is in
shade at any given time, limiting power. Several proposed designs would have more
substantial arrays with pointing mechanisms that allow the arrays to be kept
pointing at the sun. One design would
also use the solar
panel as a communications reflector to boost data rates, helping to address
the problem of deep space communication.
However, even with larger solar panels, power levels will still be
modest. One of the more capable proposed
designs, by Tethers
Unlimited, would generate 80 W in Earth orbit. At Mars, the power levels would be ~40 W to
power the spacecraft systems, instruments, and deep-space communication. That would be a tight energy budget.

Perhaps the greatest creativity in the interplanetary CubeSat
community is going into propulsion. Even
if a CubeSat simply flies by its target world, it would still need to perform
several trajectory corrections along the way.
Entering orbit around another world or rendezvousing with an asteroid
would require a more substantial propulsion system that would need to fit
within a liter or two of space. A
challenge for engineers is that their fuel must be utterly safe to launch as an
auxiliary payload with a primary spacecraft.
Carrying traditional rocket fuels that could become explosives wouldn’t
be allowed.

At least two teams are
proposing to carry water aloft
and once in flight covert the water to hydrogen and oxygen, which are excellent
traditional rocket fuels. Other
teams are developing ion engines that convert substances such as xenon into
high speed plasma jets (the same basic technology on a much, much smaller scale
as the Dawn spacecraft is using to reach the asteroids Vesta and Ceres). A JPL team has proposed a 6×6
meter solar sail that would use the pressure of sunlight for propulsion. Several teams are developing electrospray micropropulsion
systems in which a liquid is drawn up tiny capillaries and electrically charged
and expelled as high energy ions. A version
under development at JPL would simply melt tiny blocks of the element
indium to provide the liquid, creating a system that is extremely simple and robust.

An example of one set of technologies and design for an interplanetary CubeSat. Click on the image for a larger version. Credit: JPL

Once an interplanetary CubeSat reaches its target, what
measurements could it make? Some
instruments, such as magnetometers, already are small and can easily fit within
a CubeSat spacecraft. Other instruments,
such as cameras can be made quite small.
The wide-angle JunoCam camera aboard the Juno spacecraft is 1.66 kg (not
including the two kilograms of radiation shield for the Jovian
environment). (These were the best figures for these masses I could find, but may reflect early estimates.)

One proposed instrument would be a miniaturized version of an
imaging
spectrometer under development by JPL and Louisiana State University. The design would both take pictures of the
target world(s) and the spectral data collected would provide information on the
surface composition. While larger
versions of the instrument exist, development is needed to reduce its size to
fit within a CubeSat.

Where might independent interplanetary CubeSats go? Given their limitation on fuel, power, and
communications, I suspect most independent missions are likely to stay
relatively close to Earth. While some
teams will undoubtedly do a dedicated mission to orbit Mars or Venus (to show
it can be done if nothing else), many much more capable spacecraft have and
will continue to visit those worlds.
However, CubeSats could be used to scout large numbers of near Earth
asteroids both for science and to prospect for resources. Tethers Unlimited has proposed a 6U, 9 kilogram
CubeSat
called HAMMERsat that could flyby and image 15
near Earth objects in a 2.5 year mission.
The company estimates that once the first HAMMERsat is designed and
proven, subsequent copies could be flown and operated for approximately $3+M. (The
first spacecraft, however, could cost several times that figure I suspect.)

I think another likely destination will be our own moon. While many larger spacecraft have and will
visit this word, too, the moon is close by and well suited to CubeSat
missions. Just as many university
teams have flown Earth-orbiting CubeSats, I suspect a number will also target
our moon.

Another approach for independent missions would be to forego
strict adherence to CubeSat form factors.
In this approach, CubeSat components would be used and augmented with
larger, more capable components as appropriate.
Microcosm has taken this approach with their
Hummingbird design. It would carry a capable telescope that by
itself would weigh over 3 kg (almost the weight of each INSPIRE CubeSat). The attached instrument(s) could be up to 30
kg. The spacecraft would carry sufficient
fuel to enable complex missions. (See my
previous
post for JPL and NASA concepts that also would scale up CubeSat
technology to SmallSats.)

I believe, however, that the greatest potential for CubeSats may
be as auxiliary spacecraft carried to their targets by larger, more traditional
spacecraft. These CubeSats can play any
of several roles. They could carry out
measurements in locations where the mother craft can’t be. Or they could work in coordination with the
mother craft and/or possibly other CubeSats to make measurements in several or
many locations at once. And finally,
they could be sent on “suicide” missions to locations too dangerous to send the
mother craft.

As an example of the first possibility, consider a flyby
mission to Uranus. Each of Uranus’
five major moons appear to have diverse
geologic histories and two are candidates for hosting their own internal oceans
(see Oberonand Titania) and Ariel has features like Saturn’s
Dione suggesting internal processes that modified the surface. As a mother craft transits the Uranus system,
it could flyby only one moon closely enough to make detailed imaging,
composition, magnetometer, and gravity measurements. To enable measurements in
many places during the flyby, the mother craft could drop off multiple CubeSats
that would perform their own close flybys of a moon and then relay their
results back to the mother craft.

Uranus's moon Titania as seen in low resolution by the Voyager 2 spacecraft in 1986. Credit: NASA

Researchers have proposed a number of possibilities for missions
that would use Cubesats to make dispersed, coordinated measurements. One proposal would have a mother craft and
several Cubesats detect the radio waves characteristic of lightning at Venus
and triangulate the location of the bolts.
If the lightning were located preferentially over volcanoes, that would
be powerful evidence of active volcanism on Venus. Another proposal, Atromos, would modify the CubeSat form factor to enable low cost
weather stations on Mars to study processes such as dust storms from multiple
locations simultaneously. Another
concept for Mars would use multiple CubeSats to form an orbital network for
meteorological observation. (This
presentation briefly describes this
concept and provides good of background on the challenges and possible
solutions for planetary CubeSats.) Multiple
CubeSats could also deployed during a flyby of a moon,
asteroid, or comet to, for example, image the world from multiple vantage
points and enable high resolution gravity measurements via
spacecraft-to-spacecraft radio tracking.

Many possibilities exist for CubeSats sent to locations too
dangerous for a multi-hundred million dollar spacecraft. One possibility would be for a Europa flyby
mother craft to deploy CubeSats to crash onto the surface of Europa. As the CubeSats make
their plunge, they would gather high resolution images of potential future
landing sites or areas of scientific interest that they would relay back to the
mother craft. Another possibility would
be to have a CubeSat skim above Uranus on a flyby of that world for high
precision gravity measurements, a trajectory that carries considerable risk
because of that world’s rings.

While I believe that planetary CubeSats will play an
important role in future exploration, it’s important to also recognize their limitations. There are some missions where many spacecraft
making simple measurements would enable studies a single spacecraft couldn’t do
(see above). With scientific payloads of
a kilogram or two, however, the measurements CubeSats could make will be
constrained. Sending one or a hundred
CubeSats to Europa, for example, would not equal the measurements that would be
made by the 100+ kg of highly capable, synergistic instruments proposed for the
Europa Clipper mission.

Also, although CubeSats are cheap compared to traditional
planetary spacecraft, their costs will still be meaningful. If a CubeSat is intended as an integral part
of a mission’s science goals, then it must have the same reliability as the
science instruments on the mother craft.
Carrying a capable CubeSat plus its deployment device might add 15+ kg
of mass to a mission (and multiply that figure for each additional CubeSat
carried). The CubeSat design, hardware,
and testing would add several million dollars in cost. The mother craft may have to be upgraded with
a communications system and software to relay commands and data to and from the
CubeSat at a cost of additional millions of dollars. The entire system of CubeSat(s) and mother
craft would have to be extensively tested.
In the end, deciding whether to carry a CubeSat as part of the core
mission could come down to a tradeoff: carry an extra spectrometer or radar or
another instrument on the main spacecraft or carry a CubeSat or two?

One possible role of planetary CubeSats, though, especially excites
me. In Earth orbit, CubeSats have opened
space flight to university students around the world. Perhaps future planetary missions could
reserve the mass for one or two student-led CubeSats that would operate
independently of the main spacecraft, limiting the cost impacts to the primary
mission. Imagine how the opportunity to build
their own spacecraft to make measurements at Mars or a comet or wherever in the
solar system could energize the next generation of planetary scientists and
engineers.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.